1. Field of the Invention
The present invention relates generally to electrochemical fuel cells and specifically to a method of electrochemical promotion to increase the speed of carbon monoxide (CO) oxidation in hydrogen production.
2. Discussion of the Prior Art
In an effort to find alternative energy sources, hydrogen can be used in an electrochemical reaction to generate electricity. Generally, the reaction takes place in fuel cells. Fuel cells are known in the prior art for directly converting chemical energy of a fuel to electrical energy. Fuel cell advantages include low emissions, high fuel energy conversion efficiencies, and low noise and vibrations (U.S. Pat. No. 5,248,566 to Kumar, et al.) Despite the advantages, various problems are presented by existing fuel cell technology.
Proton exchange membrane (PEM) fuel cells are well known in the prior art and contain a membrane electrode assembly (MEA). The MEA has an anode compartment and a cathode compartment separated by a solid polymer electrolyte membrane. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode, and contain appropriate channels for distributing the fuel cells"" gaseous reactants.
Fuel is provided to the anode and an oxidizer is provided to the cathode; the reaction between the electrodes generates a current flow of electricity. Further, when hydrogen fuel reacts with oxygen, the reaction creates a harmless emission of water. This is certainly desirable over emissions from an internal combustion engine (ICE). The output of electrical energy from the fuel cell is dependent upon a variety of driving conditions, such as gas pressure, cell temperature, and a gas utilization ratio.
The electrodes in the fuel cell typically contain a catalyst to promote the reaction. The catalyst structure should have a low catalyst loading, efficient proton and gas access, electric continuity, low internal electric resistance, and low susceptibility to carbon monoxide (CO) poisoning. The efficiency of the catalyst is affected by contaminants that block hydrogen absorption. By way of example, CO absorbs onto catalysts, such as platinum, at temperatures below 150 degrees Celsius to prevent hydrogen absorption.
PEM fuel cells are sensitive to CO poisoning. Further, PEM fuel cells cannot operate at high temperatures. As a result, a desire exists to improve fuel cell performance to lower CO levels and allow the fuel cells to operate at a wider range of temperatures.
U.S. Pat. No. 4,910,099 to Gottesfeld, discloses an invention in which oxygen is injected into a hydrogen fuel stream ahead of a PEM fuel cell that contains CO. A surface reaction occurs (even at PEM operating temperatures below 100 degrees Celsius) to remove CO and restore electrode surface area so that the hydrogen reaction may generate current. Thus, a fuel stream from a PEM fuel cell may be formed from a methanol source using conventional reforming processes for producing hydrogen. Unfortunately, this method will reduce the fuel cell working voltage and thus reduce the system efficiency.
U.S. Pat. No. 5,248,566 to Kumar et al., describes a system in which a partial oxidation reformer is connected to the fuel tank and a fuel cell. The partial oxidation reformer receives hydrogen-containing fuel, water, and air and, in the presence of an oxidizing and reforming catalyst, produces a hydrogen-containing gas. The gas is then sent to the fuel cell negative electrode where, in combination with air sent to the positive electrode, power is produced to operate an electric motor. The invention further contains a zone where carbon monoxide, in the presence of an oxidation or methanation catalyst, is converted into carbon dioxide or methane and an afterburner unit that converts exhaust from the negative electrode of the fuel cell to heat and water.
U.S. Pat. No. 5,336,570 to Dodge, Jr., discloses a hydrogen fuel cell that obtains improved breathability and hydrogen sealing. The proton exchange membrane in the fuel cell is clamped between two catalytic electrodes. Although this prior art deals with hydrogen power cells, the invention does not relate to CO removal.
Prior art reveals other methods, such as selective oxidation, to reduce CO concentration. U.S. Pat. No. 5,482,680 to Wilkinson et al., discloses a method for using selective oxidation, within the fuel cell itself for removing carbon monoxide present in the incoming reactant stream. The catalyst selectively oxidizes carbon monoxide, contained in the fuel stream passageway, to carbon dioxide. Carbon monoxide produced by a reverse water-shift is also oxidized. Additionally, U.S. Pat. No. 5,432,021 to Wilkinson et al., reveals another method for selective oxidation. This method and apparatus oxidizes the carbon monoxide present in the incoming fuel stream and/or present in the reverse water shift to carbon dioxide. U.S. Pat. No. 6,010,675 to Trocciola et al., discloses a method and apparatus for removing CO from a gaseous media. The CO concentration is selectively reduced by selective catalytic oxidation. The oxidation occurs in the presence of gaseous oxygen by passing the gaseous medium through a catalyst. The catalyst oxidizes the CO in an endothermic reaction at a controlled temperature.
Unfortunately, the above-mentioned selective CO oxidation or water shifting reaction is slow to respond to load changes. The present invention proposes using electrochemical promotion to improve CO removal in a fuel cell system.
Accordingly, an object of the present invention is to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal from electrochemical promotion.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal that is easy to implement.
It is a further object of the present invention to provide a CO removal method and system for pure hydrogen production with a fuel reformer, based upon non-Faradaic electrochemical modification of catalyst activity (electrochemical promotion). By applying a potential, or small current, on the catalyst, catalytic activity can be greatly enhanced.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal that, with proper application on the catalyst, the catalyst surface working function can be changed and thus the catalytic reaction rate can be changed. This provides a new and useful approach to increase catalyst work efficiency for CO removal and thus reduce fuel cell system size.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal that reduces catalyst working temperature by increasing the catalytic reaction rate. Typically catalysis reaction rate is quite low for CO removal except at relatively high temperatures. The enhanced catalysis rate will make the catalyst high working temperature unnecessary.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal that assures rapid start-up and excellent dynamic response by reducing its working temperature and increasing catalysis reaction efficiency.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal that can precisely control CO removal by varying the applied current or potential on the working electrode.
It is a further object of the present invention to provide an improved fuel cell power plant with electrochemical enhanced carbon monoxide removal where catalysts can include Pt, Rh, Au, Cu/ZnO, Cu/CuO, ABO3 (perovskite), zeolite, and Pd, but not limited to these catalysts.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.